This invention relates to base-catalyzed self-crosslinking polymers which can be cured at low temperatures. It also relates to the use of water-soluble, cationic, base-catalyzed self-crosslinking polymers as electroconductive coatings for paper substrates.
It is well-known in the art to employ self-crosslinking polymers, either in emulsion or solution form, as coatings, binders, or adhesives for a variety of substrates. Self-crosslinking polymers are distinguished from crosslinkable polymers in that the cross-linkable polymers contain a functionality, such as a carboxyl group, which can only be crosslinked by the addition of a co-reactant (i.e., crosslinker) to the polymer emulsion or solution. A typical crosslinkable system can be represented as follows: ##STR1##
In contrast, self-crosslinking polymers contain a functionality which is self-reactive and consequently do not require the use of a coreactant species per se. A typical system can be represented as follows: ##STR2## The advantages of self-crosslinking polymer systems are their simplicity, economy, and particularly their efficiency. Such systems have been used as textile adhesives, non-woven binders, pigment binders for glass fabrics, and fabric finishing agents for hand and weight modification. On curing, such systems produce textile products with excellent durability to washing and dry cleaning. They have also been used in pigment printing and dyeing and as a binder for paper.
Most of the self-crosslinking and co-reactant crosslinkable polymer systems of the prior art suffer from the disadvantage that toxic, free formaldehyde is present either during the curing or the preparation of the polymers. The self-crosslinking systems, which are typically formaldehydeamide polymeric adducts containing methylolacrylamide repeating units, liberate formaldehyde during curing of the crosslinked thermoset polymer. The crosslinkable systems, which are typically based on urea-formaldehyde or melamine-formaldehyde resins and crosslinkers, may contain residual free formaldehyde. In addition to the odor problems created by the presence of free formaldehyde, the dermatitic effect may present a serious problem. The exposure of operating personnel and consumers to formaldehyde has been a recent concern of both industry and regulatory agencies.
A further disadvantage of the above self-crosslinking systems is that they require an acidic cure. When used as electroconductive coatings on paper substrates, the acid tends to degrade the paper and cause yellowing.
This has lead to the search for formaldehyde-free systems, especially formaldehyde-free self-crosslinking systems which can be crosslinked at low temperatures, preferably at room temperature, by the addition of a base rather than an acid.